This invention relates generally to the field of radar. More specifically, the present invention relates to synthetic aperture radar (SAR).
The operation of SAR is a side-looking process. This is due to the fact that one axis of the imaging process is the radar range, and the imaging process effectively collapses in the range direction of the antenna aperture which is downward looking. Any response from energy in the downward direction is eliminated by range gating (i.e., the elimination of the signal by exclusion of certain time periods in the receive window). For many microwave SAR systems, the antenna pattern is directional enough that the combination of range gating and pattern amplitude is sufficient for good imaging.
Microwave SAR systems are, however, limited in their capability to identify targets through obstruction such as foliage. Therefore, one must look to lower frequency imaging systems for effective penetration. On the other hand, VHF SAR is useful because it is low enough in frequency to operate through foliage. But, as the frequency of operation drops to the VHF range, the size of the typical antenna is electrically too small to achieve a pattern that is very directional, therefore range gating no longer suffices to ensure good imaging.
The fundamental problem lies in the fact that electrically small apertures produce near omni directional beam patterns. Omni directional beam patterns cause a multitude of reflection back into the radar at the same ranges as the desired target. These are impossible to eliminate by range gating because they will pass through the same range gate as the intended target image. What is lacking in the prior art is either a method or apparatus which can provide good SAR imaging at lower (i.e., VHF) frequencies.